In reply to EZ:
> There are obvious implications for credit referencing that I don't care about here.
Surely there are both pros and cons regarding that, for example if wrongly given a bad credit rating based on incorrect information, it could be used as proof on your behalf?
Back to the topic in hand, though, isn't it similar to the situation regarding the census?
I think you are more likely to get into trouble for giving deliberately false information, rather than not giving information at all.
http://en.wikipedia.org/wiki/Electoral_Register#United_Kingdom says
A fine of up to £1,000 (level 3 on the Standard scale) can be imposed for giving false information.
which is different from not supplying any information at all, in my view...